The invention relates to methods and systems for obtaining ellipsometric and reflectance measurements of a small region of a sample over a range of UV (and preferably also visible) wavelengths, and optionally also for determining, from the measurements, the thickness and refractive index of a very thin film on the sample. The sample can be a semiconductor wafer having at least one thin layer over a silicon substrate. Preferred embodiments of the invention include both a spectrophotometer and an improved spectroscopic ellipsometer which share a common focal point on the sample and preferably a common radiation source.
Among the well known nondestructive testing techniques are the techniques of spectroreflectometry and spectroscopic ellipsometry, which measure reflectance data by reflecting electromagnetic radiation from a sample. In spectroscopic ellipsometry, an incident radiation beam having a known polarization state reflects from a sample (generally at high incidence angle), and the polarization of the reflected radiation is analyzed to determine properties of the sample. Since the incident radiation includes multiple frequency components, a spectrum of measured data (including data for incident radiation of each of at least two frequencies) can be measured. Typically, the polarization of the incident beam has a time-varying characteristic (produced, for example, by passing the incident beam through a mechanically rotating polarizer), and/or the means for analyzing the reflected radiation has a time-varying characteristic (for example, it may include a mechanically rotating analyzer). Examples of spectroscopic ellipsometry systems are described in U.S. Pat. No. 5,329,357, issued Jul. 12, 1994 to Bernoux, et al., and U.S. Pat. No. 5,166,752, issued Nov. 24, 1992 to Spanier, et al.
In the technique of spectroreflectometry an incident radiation beam reflects from a sample, and the intensity of the reflected radiation is analyzed to determine properties of the sample. The incident radiation includes multiple frequency components (or is monochromatic with a time-varying frequency), so that a spectrum of measured data (known as a reflectance spectrum or relative reflectance spectrum) including data regarding reflected intensity of incident radiation having each of at least two frequencies is measured. Systems for spectroreflectometry are described in U.S. Pat. No. 5,241,366 issued Aug. 31, 1993 to Bevis et al., and U.S. Pat. No. 4,645,349, issued Feb. 24, 1987 to Tabata, and the following U.S. patent applications assigned to the assignee of the present invention: U.S. Ser. No. 07/899,666, filed Jun. 16, 1992 (abstract published on Apr. 26, 1994 as the abstract of U.S. Pat. No. 5,306,916), and pending U.S. Ser. No. 08/218,975, filed Mar. 28, 1994.
Reflectance data (measured by spectroscopic ellipsometry, spectroreflectometry, or other reflection techniques) are useful for a variety of purposes. The thickness of various coatings (either single layer or multiple layer) on a wafer can be determined from spectroscopic ellipsometry data (indicative of the polarization of radiation reflected from the sample in response to incident radiation having known polarization state), or a reflectance spectrum or relative reflectance spectrum.
The reflectance of a sample (or sample layer) at a single wavelength can be extracted from a reflectance or relative reflectance spectrum. This is useful where the reflectance of photoresist coated wafers at the wavelength of a lithographic exposure tool must be found to determine proper exposure levels for the wafers, or to optimize the thickness of the resist to minimize reflectance of the entire coating stack.
The refractive index of a coating on a sample (or layer thereof) can also be determined by analysis of spectroscopic ellipsometry data (indicative of the polarization of radiation reflected from the sample, in response to incident radiation having known polarization state) or an accurately measured reflectance spectrum.
It would be useful for a variety of industrial applications to determine the thickness of a very small region of a very thin film (less than 30 angstroms in thickness) on a substrate from reflectance measurements (with sub-angstrom measurement repeatability) of the sample (e.g., where the sample is a semiconductor wafer and the very thin film is coated on a silicon substrate of the wafer). It would also be useful for a variety of industrial applications to obtain reflectance measurements using a single measurement system, and then analyze the measured data to determine the refractive index and thickness of a layer of a sample, where the layer has unknown thickness in a broad range from more than 10 microns to less than 10 angstroms.
It would also be useful to obtain reflectance measurements using a single measurement system, and then analyze the measured data to determine the refractive index and thickness of any selected layer of a multiple layer stack (where each layer has unknown thickness in a range from more than 10 microns to less than 10 angstroms). Such multiple layer stacks are often produced during the manufacture of semiconductor integrated circuits, with the stacks including various combinations of material such as SiO2, Si3N4, TiN, Poly-Si, and a-Si.
Because of the tight tolerance requirements typically required in the semiconductor arts, an extremely accurate method and apparatus (e.g., having sub-angstrom repeatability) is needed for determining film thickness and refractive index measurements from reflectance data from a very small, and preferably compact region (e.g., a microscopically small region of size less than 40 micronxc3x9740 micron) of a wafer. However, it had not been known how to accomplish this using an ellipsometer with all-reflective optics (for use with broadband UV radiation). Conventional ellipsometers had employed transmissive optics to direct a beam at a sample, either with relatively high incidence angles (angles substantially greater than the zero degree incidence angle of xe2x80x9cnormallyxe2x80x9d incident radiation at a sample) as in above-cited U.S. Pat. No. 5,166,752, or with low incidence angle (normal or nearly normal incidence at the sample). The inventors have recognized that such transmissive optics are unsuitable for use with broadband radiation of ultraviolet (or UV to near infrared) wavelengths, and have also recognized that beams of such radiation incident on reflective ellipsometer components with high incidence angles undesirably undergo a large change in polarization upon reflection from each such reflective component. The inventors have also recognized that the change in beam polarization upon reflection from each optical component of an ellipsometer should be small relative to the polarization change (due to specific properties of the sample itself) occurring on reflection from the sample, and that such small polarization changes can be achieved by reflecting an ellipsometer beam from optical components of an ellipsometer only at small incidence angles (where the ellipsometer reflectively focuses the beam to a small, compact spot on the sample, with rays of the beam incident at the sample at a substantial range of high incidence angles).
Until the present invention, it had not been known how to meet the needs set forth in all three preceding paragraphs, and avoid the described limitations of the prior art set forth in these three preceding paragraphs.
The spectroscopic ellipsometry method and apparatus of the invention employs reflective optics to measure a small (and preferably compact) region of a sample (e.g., a microscopically small, square-shaped spot on the sample) by reflecting broadband radiation having a range of UV (and preferably also visible and near infrared) wavelengths from the region. The method and apparatus of the invention optionally also determines from the measurements the thickness and/or complex refractive index of a thin film on the sample (such as a layer of a multiple layer stack over a silicon substrate of a semiconductor wafer). Preferred embodiments of the inventive ellipsometer employ only reflective optics (along the optical path between the polarizer and analyzer) to avoid aberration and other undesirable effects that would otherwise result from transmission of broadband ultraviolet (UV) radiation through transmissive optics, and direct the beam so that it reflects with low incidence angle from each such reflective optical component. Preferred embodiments of the inventive ellipsometer focus a beam having elongated cross-section from an elliptical focusing mirror to a small, compact spot on the sample at a range of high incidence angles. The elliptical shape of the mirror surface reduces off-axis aberrations such as xe2x80x9ccomaxe2x80x9d in the focused beam. Use of a reflective focusing element (rather than a transmissive lens) eliminates chromatic aberration in the focused beam.
Preferred embodiments of the invention include a spectrophotometer and an improved spectroscopic ellipsometer integrated together as a single instrument. In such integrated instrument, the spectrophotometer and ellipsometer share a broadband radiation source, and radiation from the source can be focused by either the spectrophotometer or the ellipsometer to the same focal point on a sample. Some of these embodiments include means for operating a selected one of the spectrophotometer and the ellipsometer. Others of the embodiments include means for supplying a portion of the radiation from the source to each of the spectrophotometer and ellipsometer subsystems, thus enabling simultaneous operation of both subsystems to measure the same small sample region.
Preferred embodiments of the inventive ellipsometer reflect a beam from a focusing mirror (where the beam has low incidence angle at the mirror) to focus a beam onto a small, square-shaped spot on a sample with high incidence angle. Preferably, the beam focused onto the spot has a substantial range of high incidence angles (e.g., the beam is a converging beam whose rays are incident at the sample with incidence angles in the range from about 63.5 degrees to 80.5 degrees), and a means is provided for selectively measuring only a portion of the radiation reflected from the sample after being incident at a single, selected high incidence angle (or narrow range of high incidence angles). In preferred implementations of these embodiments, a beam having elongated cross-section is focused from an elliptical focusing mirror to a compact spot on the sample, and the numerical aperture of the focusing mirror is sufficiently large to focus the reflected beam with a desired (sufficiently large) range of high incidence angles.
Preferred embodiments of the inventive ellipsometer also employ a rotating, minimal-length Rochon prism to polarize the broadband radiation beam incident on the sample (and also employ a fixedly mounted analyzer). The prism preferably has only the minimum length needed to enable the beam to pass through its clear aperture, because the prism""s length is proportional to the amount of chromatic aberrations introduced by the prism. Alternatively, a phase modulator can be substituted for a rotating polarizer, or a fixedly mounted, minimal-length polarizing element can be employed with a rotating analyzer.
Other preferred embodiments of the inventive ellipsometer include a spectrometer which employs an intensified photodiode array to measure reflected radiation from the sample. Each photodiode in the array measures radiation, having wavelength in a different range, reflected from the sample. The intensified photodiode array may include an intensifier means, which preferably includes a top photocathode surface which emits electrons in response to incident photons, means for accelerating the electrons to a bottom phosphor surface, and a fiber optic coupler for directing photons emitted from the bottom phosphor to the photodiode array.
In some embodiments, the inventive ellipsometer includes a reference channel (in addition to a sample channel which detects radiation reflected from the sample). Illuminating radiation from the source is split into a sample beam and a reference beam, preferably by a bifurcated optical fiber. The sample beam reflects from the surface of a sample and is directed to the sample channel detector. The reference beam does not reflect from the sample, but is directed to the reference channel detector. By processing reference signals from the reference channel detector, as well as signals from the sample channel detector, the thickness of a very thin film on the sample (or the sample""s refractive index) can be more accurately determined.
The invention has many applications, such as measuring refractive indices, measuring film thicknesses, and determining lithographic exposure times, and (in embodiments including a spectrophotometer) measuring reflectance spectra.